The vast majority of human neoplasms is thought to be due to environmental factors. There is overwhelming evidence that skin cancer, one of the most commonly occurring human cancers, is usually due to exogeneous insult. The cells that are most likely to be progenitor cells for this carcinoma are the basal cells, which are rapidly dividing and/or differentiating, and as such make an excellent target cell for carcinogenic agents. The purpose of this work is (1) to develop an in vitro system consisting of dividing, basal cells isolated from epidermis, (2) to induce these cells to undergo neoplastic transformation in vitro following treatmnt with environmental carcinogens and/or promoters, (3) to determine whether the two-stage model of carcinogenesis is a universal phenomenon, common to animals other than the mouse, (4) to isolate cells from skin papillomas and carcinomas induced in vivo, and (5) to compare the cellular properties of epidermal cells at various stages after in vitro treatment with carcinogens to cells derived from papillomas and carcinomas. The work will be done with inbred Syrian hamsters.